1. Field of the Invention
The present invention relates generally to a process for plastic article shaping. In particular, it relates to the recycling of reclaimed and purified process material, especially of extrudant-receiving bath material.
2. Prior Art
In the continuous production of filamentary vinyl polymer compositions by the wet spinning technique, it is well-known, common practice to polymerize a chosen vinyl monomer composition in a concentrated aqueous inorganic salt solution and to spin the resulting vinyl polymer solution into a coagulating bath which contains an aqueous solution of the same inorganic salt as that employed in the polymerization step. In the coagulation bath, the concentration of inorganic salt is maintained at a level which affords precipitation of the vinyl polymer to form a coherent filamentary gel. The vinyl polymer filamentary gel is then washed, and the inorganic salt is recovered as a concentrated solution from both the washing and coagulation baths by concentrating the effluents thereof. Such concentrated aqueous inorganic salt solution is recycled into the polymerization step for the purpose of economy of operation of the process. See e.g. U.S. Pat. Nos. 2,916,348 and 2,558,730.
However, in the recovery of the inorganic salt from washing and coagulation bath effluents, unreacted monomer, low molecular weight polymer, and a number of other organic molecular impurities, as well as metallic ion impurities accumulate, adversely affecting solution polymerization activity in addition to the final quality of the filamentary vinyl polymer product.
As a consequence, a number of processes for purifying the recovered concentrated inorganic salt solution prior to the recycling thereof have been proposed. Exemplary of the more noteworthy of these processes are those described and claimed in U.S. Pat. Nos. 3,702,880 and 3,107,971; and Japanese Pat. Nos. 16,932/67 and 27,248/67. Of these the most generally applicable and most highly efficacious processes are those which comprehend an oxidative treatment employing oxidation agents such as hydrogen peroxide, water-soluble persulfates, sodium hypochlorite, sodium chlorate or perchlorate, and potassium permanganate. Howsoever effectual these and like oxidative processes might be, they are found wanting in view of one or more of the following disadvantages: (a) Vinyl monomers are capable of being oxidized to oxalic acid and oxalates, which form precipitates with certain metallic ions, notably zinc, thereby interfering with both the solution polymerization and the spinning of the polymer solution. (b) The introduction of cations such as Na.sup.+ and K.sup.+ can result in the formation of precipitates such as MZnCl.sub.3 when concentrated aqueous solutions of the inorganic salt ZnCl.sub.2 are cooled. (c) The employment of permanganate oxidizing agents results in the undesirable introduction into the system of manganous ion, which is both a chain terminator and a chain transfer agent, and as such seriously interferes with the vinyl polymerization. (d) Certain organic impurities present in the aqueous inorganic salt solution are oxidized to form complexing agents, which significantly affect the rate of vinyl polymerization.